Hair coloring compositions are used to color human hair. They are used for a variety of purposes; most commonly to return gray hair to its previous color, to change hair color to a shade regarded as more desirable, or to return hair to its original color after chemicals or environmental factors (e.g. tints, relaxers, sun bleaching) have discolored it.
Hair coloring compositions are typically categorized based on the length of time they effect the colored hair. The four most common classifications are “temporary,” “‘semi-permanent,” “demi-permanent,” and “permanent.”
The hair dye molecules in temporary hair colorings are large and, therefore, do not penetrate the cuticle layer, allowing only a coating action that may be removed by shampooing. Semi-permanent hair colorings usually include smaller hair dye molecules than those of temporary hair colorings, and are therefore able to penetrate the hair cuticle and reach the hair cortex.
Demi-permanent hair colorings usually last longer than the semi-permanent variety. The hair dye molecules in demi-permanent compositions enter the hair through the cuticle, as in the semi-permanent compositions, but once in the hair cortex they fuse with other molecules to give medium-sized color molecules. Because these molecules are larger in size, they now take much longer to wash out of the hair. Permanent hair coloring compositions are used for major hair color changes, for example going from black to blonde or vice-versa. When the hair dye molecules of permanent hair colorings enter the hair they react and expand to a size that cannot actually be washed out.
In order to increase the uptake of hair dye molecules into hair, hair coloring compositions may include a hair swelling agent. Without being bound to any particular theory, it is believed that the swelling agent increases hair swelling sufficiently to raise the cuticle of the hair shaft so that the color can be deposited therein.
Conventional swelling agents include, for example, ammonia, urea, thiourea, acetic acid, phosphoric acid, formic acid, formamide, ethyl amine, alkali halides such as potassium iodide, sodium bromide, lithium bromide, and lithium chloride. Ammonia is the most commonly used swelling agent. Unfortunately, conventional swelling agents are often characterized by an unpleasant odor and/or can be harsh and damaging to hair. This is especially true if the hair is dyed on a frequent basis.